This greatly depends on your photographer, and their experience level and shooting style.
Upon arrival, the photographer will need between 5 minutes and 60 minutes to set up their equipment and do some test shots. For simple cosplay shoots this is usually pretty quick. For high-production commercial shoots this can be a ton more complex and time-consuming (maybe half a day to set up). Take this time to do final adjustments to your costume and makeup. If you're there with a friend, go over each others' costumes in detail to make sure everything is in place (side note, take a repair kit with safety pins, bobby pins, double-sided tape, etc). Don't expect the photographer to "just photoshop it" or "fix it in post." Take two minutes or ten minutes and get it perfect there on the set, this will be WAY BETTER than trying to 'shop it later.
The photographer will likely ask you to stand in for some test shots. You don't have to "pose" for these, you can just stand there and relax or you can practice and warm up if you like. I'll usually make silly out-of-character faces and poses to lighten up a bit and break the ice.
After that, you have as much time as you need (and have previously agreed upon) to run through as many poses and shots as you like. If you brought a pose reference sheet, go over it with the photographer at the beginning of the session, and make a plan for the poses.
The photographer will likely get a good mix of of full-body, half-length, and face/bust shots. They'll likely tell you how close-up they're shooting, so you know how much of your body to pose. The photographer may be skilled at giving pose direction ... or they may not, in which case it's on you to do your own posing. DON'T FORGET FACIAL EXPRESSIONS - hopefully you've been practicing these as well. Almost as a rule, exaggerated poses and exaggerated facial expressions make fore more interesting photos
Ask the photographer if they will tell you when to change poses. Some photographers like to take several shots of the same exact pose .. I usually don't; it's usually not necessary. instead, every time you hear the shutter click (or see the flash go off), change your pose a little or a lot. It doesn't have to be a drastic change - for instance, just look in a different direction with your eyes, or keep the same body pose but change facial expressions. Or change the position of your prop, or your hands. Or change your body pose entirely. Being able to change poses yourself, instead of waiting for direction for each shot, will help the "flow" of the shoot. Here's a video of a skilled professional model - see, every time the flash fires, he gives a new pose - note, it takes years of experience to get this good, so don't expect to be this fast..
If you're doing poses with another person, you'll almost always want to pose much closer together
than your normal comfort zone. get in each others' personal space; it makes for way better photos. If you keep to your normal personal-space boundaries, you look miles apart in the photo.
If you have a pose/shot list, do all the less strenuous ones first, so your makeup doesn't wear off, you don't get sweaty, and your costume/props stay in good shape. Once you have everything on your shot list done, you might try some more elaborate or strenuous poses, action shots, etc.
If you change locations and setups, the photographer may need to do another round of fine-tuning camera settings and test shots. At this time, go over your costume and props and hair again, make sure everything is perfect.
When you're done, discuss with the photographer what you should expect next - of course, you should have already done most of this before hiring the photographer. Will you get proofs (and when), how many finished shots, what level of retouching or editing to expect, when can you expect to receive them, by what method (usually email or dropbox), what rights you have for further usage (online sharing, printing, entering into contests, etc).
Some additional reading: